Lighting is the second largest end-use of energy in buildings. Energy consumption for all lighting in the U.S. is estimated to be 8.2 quads, or about 22% of the total electricity generated in the U.S. According to the Department of Energy Solid State Lighting Program Multi-Year Program Plan, more than half of these 8.2 quads are consumed in the commercial sector, the largest energy user for lighting. Nationally, in the residential and commercial sectors, lighting constitutes over 30% of total electricity use. Inefficient lighting not only impacts the electricity directly consumed due to lighting, it also affects overall electricity consumption because inefficient lighting sources contribute to a building's internal heat generation and subsequent air-conditioning loads. In both the residential and commercial sectors, incandescent light bulbs account for almost 50% of the sources of lighting. These incandescent light bulbs are extremely inefficient, with more than 90% of the energy consumed contributing to heat generation. Therefore, the actual amount of electricity consumed due to light generation is substantially higher than 22%.
Organic Light Emitting Devices (OLEDs) have demonstrated very high efficiency. Recently, several groups have demonstrated that OLEDs can achieve a luminous efficiency exceeding 100 lm/W, which corresponds to about 7 times the efficiency of conventional incandescent light bulbs and 2 times the efficiency of fluorescent light tubes. OLEDs are extended light sources that do not require luminaries, but these devices are often still attached to the power-grid.
Organic solar cells are promising as a new source of renewable energy. Organic solar cells are attractive for the next generation photovoltaics because of their compatibility with flexible substrates, low manufacturing costs, and large area applications. Specifically, organic solar cells made with polymers can be printed on plastic substrates at very low costs by roll-to-roll processes. Typically, solar cells are semi-transparent instead of “black” compared with typical inorganic solar cells. The color appearance of polymers can also be varied by modifying their molecular design. As a result, organic solar cell-coated windows can be used for energy harvesting as well as for window tinting applications. However, a large area is often used for solar cell applications and the power generated is typically converted for immediate use or efficiently stored.
Thus, an off-grid lighting solution that maximizes energy storage and conversion efficiency, while fitting into currently available building space, can be useful.